This invention relates to a gaseous fluid flow meter utilizing a Karman vortex street.
Gas flow meters utilizing the Karman vortex street include a vortex generating rod immersed in a gaseous fluid flowing through a conduit perpendicularly to the direction of flow of the fluid to generate the Karman vortex street downstream of the rod and employ an ultrasonic wave to detect the Karman vortex street thereby to measure a flow rate of the gaseous fluid. In order to detect the Karman vortex street, it has been proposed to dispose an ultrasonic transmitter and an ultrasonic receiver in opposed relationship in the conduit through which a measured gaseous fluid flows so that an ultrasonic wave transmitted from the ultrasonic transmitter is modulated by the vortices of the Karman vortex street and then received by the ultrasonic receiver. The ultrasonic wave is continuously transmitted to the receiving side and the modulation is a change in phase of the ultrasonic wave due to vortices is first sensed. Such a system is, on the one hand, advantageous in that when the energy of the ultrasonic wave is increased, the output from the receiving side becomes high and influence of multi reflection and diffused reflections within the conduit can be fully disregarded. On the other hand, the arrangement is disadvantageous in that a standing wave is generated between the opposed transmitting and receiving elements due to resonance, and this standing wave has a node coinciding with the position where the receiving element is mounted. This affects the change in phase due to the effect of the vortices, which in turn makes the sensing of an accurate flow rate difficult or impossible. Particularly, when the flow rate of air is being measured, the ultrasonic wave is propagated through the air at a propagation velocity which changes with a change in the air temperature. If the ultrasonic wave being used has a constant frequency, then this change in propagation velocity is attended by a variation in the wave length thereof. This has resulted in the disadvantage that a standing ultrasonic wave is formed at a certain temperature of the air which causes the ultrasonic receiver not to receive in a normal fashion the ultrasonic wave transmitted from the ultrasonic transmitter.